1. Field of the Invention
The invention relates generally to methods and systems for carrying out adsorption processes and more particularly to a non-swing, meso-frequency traveling wave electro-kinetic system capable of use in purification/separation and/or refrigeration/heat pump processes.
2. Discussion of the Related Art
Adsorption is a process by which a gas or dissolved material is assimilated onto the surface of a solid or liquid material and defined in terms of adsorptive surface area per unit volume. In contrast, an absorption process entails incorporation of materials into the pores or interstitial spaces, as opposed to only the surface, of an absorbent material. An adsorbing material is called an adsorbent or sorbent. A material being adsorbed (or sorbed) is called the adsorbate or sorbate.
A number of different factors and mechanisms influence the adsorption process. For example, polar molecules are often more easily adsorbed. Similarly, molecules with small kinetic diameters can be preferentially adsorbed relative to molecules with larger kinetic diameters. Additionally, the condensation characteristics of the sorbate can also affect the adsorption process. Accordingly, adsorption systems can manipulate these mechanisms to separate components of complex mixtures and/or to effect selective vapor condensation.
A simple, traditional adsorption system has two separate vessels filled with adsorbent material. A complex compound is passed through the adsorbent material of one of the vessels causing a component of the complex compound to be removed from the feed stream. Once the adsorbent in the first vessel is no longer able to adsorb any more material, the feed stream is switched to the second adsorbent containing vessel. While the second vessel is adsorbing, the first vessel is being purged (i.e. desorbed) of the adsorbed material. Thereafter, the first vessel is substituted for the second vessel while the second vessel is purged. This process, known as “Swing Adsorption,” is repeated as needed.
The material handling capacity of such adsorption systems depends on a number of variables, including vessel size (i.e. adsorbent mass), cycle time and operating pressure, as well as adsorbent/adsorbate affinity. For example, increasing vessel size, and hence the volume and mass of adsorbent, increases adsorption capacity. Similarly, decreasing the cycle time provides a concomitant increase of available adsorption sites per unit time. Increasing the operating pressure of the system also increases adsorption capacity per unit volume.
Liberation of the sorbed material from the adsorbent (i.e. desorption) can occur via a number of different mechanisms. Conventional adsorption systems employ either pressure reduction or temperature increase for removal of the adsorbate. Systems swinging between adsorption and pressure differential desorption are known as Pressure Swing Adsorption (PSA) systems. Alternatively, adsorption systems switching between adsorption and temperature differential desorption are known as Temperature Swing Adsorption (TSA) systems. Other desorption mechanisms exist, including electrical energy desorption (for dielectric and/or conductive adsorbents) and microwave irradiation of adsorbent/adsorbate complexes.
Regardless of the adsorption/desorption process employed, these systems require that an energy balance be maintained in the system. That is, energy that is dissipated during adsorption (as heat) must be reintroduced into the system during desorption. The most efficient adsorption systems, in terms of energy, are those containing the least amount of superfluous mass because heating and cooling a large vessel, a large volume of adsorbent and associated binder materials during the repeating cycles is a very wasteful process. As a result, the current trend is to toward lower mass, rapid cycle systems despite the fact that such measures have traditionally been associated with reducing adsorption efficiency.
Recent advances in the field of micro electromechanical systems (MEMS) research has led to proposals for incorporating micro-channel adsorption and reaction devices that provide for very short cycles with increased heat transfer capacities into traditional PSA and TSA systems. Such devices alternate the flow and pressure of complex compounds into and from adsorbent filled micro-channels (thus increasing surface volume with minimal effect on system size). For example, corrugated sheets have been impregnated or covered with thin layers of such adsorbent materials. Additionally, such systems offer the possibility of exceedingly short cycles times on the order of tenths of seconds. Accordingly, it is envisioned that such devices would be particularly well suited for use in small devices, such as oxygen enrichment systems for hospital patients.